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Beam-foil lifetime measurements of nitrogen in the vacuum ultraviolet spectral range
Physica 62 (1972) 104-108 0 North-H&and Publishing Co.
BEAM-FOIL
LIFETIME
MEASUREMENTS
IN THE VACUUM ULTRAVIOLET
OF NITROGEN
SPECTRAL
RANGE
P.D. DUMONT Universite’ de Litige, Institut de Physique Nucliaire, Sart-Tilman - 4000 LiPge 1, Belgique
Received 21 February 1972
Synopsis Nitrogen spectra between 650 and 2OOOA have been observed for particle energies between 0.3 and 2 MeV. Lifetimes of 24 excited levels are given.
1. Introduction. The foil excitation technique allows us to obtain lifetimes of excited states of atomic elements1*2). This paper describes beam-foil measurements of nitrogen-multiplet lifetimes in the wavelength range 650 to 2000 A ; in this wavelength region only a few experiments have been carried out previously2*3,4). The beam-foil experiments were performed at beam energies between 300 keV and 2 MeV. 2. Experimental procedure. The experimental arrangement is shown in figs. la and lb. The 2 MeV Van de Graatf accelerator of the “Institut de Physique Nucltaire - Universite de Liege” is equipped with a conventional radio-frequency ion source. After traversing the magnet, the analysed ion beam iv passed through two collimator holes (8 mm and 9 mm in diameter), and enters the “excitation chamber” which contains the carbon foil*. An externally controlled mechanical system allows the foil to be moved along the ion beam. The light emitted perpendicular to the beam is measured with a Seya-Namioka vacuum ultraviolet spectrometer designed at the “Institut d’Astrophysique - Liege”. The spectrometer entrance slit is located in the excitation chamber, 2 cm from the ion-beam axis. Slit widths between 0.1 and 0.3 mm were generally used. The spectrometer is equipped with a 1200 l/mm concave grating which has focal length of 0.5 m and is blazed at 1700 A in the first order. The photocathode of the photomultiplier (P.M.) tube (ASCOP E.M.R. 541-N-05M-14) is cov* It in fact contains 11 carbon foils, 10 mm in diameter, 10 yg/cm2 thickness, mounted on a rotating disc for the purpose of quick interchange. 104
BEAM-FOIL
LIFETIME
MEASUREMENTS
OF NITROGEN
105
ered with sodium salicylate9). The P.M. tube is connected to a d.c. amplifier, followed by a strip-chart recorder. The residual pressure in the excitation chamber is kept lower than 10e6 torr’ by an oil diffusion pump trapped with liquid nitrogen. The excitation chamber, electrically isolated, is in fact a big Faraday cup, and hence we are able to obtain the ion-beam current (relative values). This current (appr. 5 PA) is continuously recorded for the monitoring purpose.
ion beam
@rJ source
il
Van de Graaff accelerator
Fig. 1 a
.
SOP - Namiika sveclromolor
Beam foil spectroscopy_ -,L&z;rk
Fig. 1 b
‘:7,3. Experimental results. 3.1. Wavelengths. Spectra of the light emitted perpendicular to the beam were recorded at constant ion current, with a mass-28 beam. Wavelength calibration was obtained by observation of lines of known * 8 x lo-’ torr, measured with Leybold’s “Ionisations-Vakuummeter
1M;VI”.
106
P. D. DUMONT
oa
I
z 0
Fig. 2
Fig. 3a
Fig. 3b
BEAM-FOIL
LIFETIME
MEASUREMENTS
OF NITROGEN
107
origin (e.g., Ly LY,C I 1561 A, . . .). Line identifications were obtained with the aid of nearby lines whose wavelengths were listed in the literature4*5), and with the intensity variations of lines with energy6). The spectra showed many known nitrogen lines (N I-N V) as well as many unreported lines; some of these we identified as C I and C II lines, perhaps from carbon atoms ejected from the foil’). Fig. 2 is an example of such a spectrum. TABLE I 2
Ion
(A)
Energy
T
z
(ns) our results
W Other results
Transition
observed (MW
1134
NI
7.6 + 0.7
7.2 + 0.7”; -7.1b
2ps%J-2p44P
1200
NI
2.35 + 0.2
2.35c; 2.4O; 2.5=
2ps w-2p2
1242
NI
2.27 + 0.2
2.6a; 2.2=; 2.65”
2p3 ‘Do-3s
776
N II
0.33 + 0.05
0.6b
836
N II
0.51 f 0.07
0.57b
2p3 3Do-2p2 3s 3P
0.5
901
N II
0.24 + 0.035
0.65b
2p3 3D0-2p 4p 3P
0.5
916
NII
0.84 f 0.08
0.77b
2p2 JP-2ps 3PO
1345
N II
1.46 + 0.2
1.7c; 2.1s
686
N III
0.15 * 0.03
0.17 * 0.03d; 0.22b
772
N III
0.25 f 0.05
0.51b 0.80b
2p2 ‘D-2p3
2p3 jD”-3p
0.5 3s 4P
2D lD”
3D
2s2 2p ZPO-2s 2p2 *P
0.5 0.5 0.5; 1.5
0.5; 1.5 1.5 1.5
\ 2s 2p2 4P-2pJ %O \ 2s 2p2 ‘D-2p3 2Po
1.5
2s 2p2 2D-2p3 *Do
1.5 1.5
980
N III
0.79 + 0.08
991
N III
2.6 f 0.3
2.2b; 2.4 + 0.24d
2s’ 2p ‘PO-2s 2p2 *D
1184
N III
0.44 + 0.05
0.4b; 0.45 + 0.1a;
2s 2p* ZP-2p3 2PO
1.5; 0.5
0.4Sc 1324
N III
1.32 + 0.26
1.6 + 0.1”
3p 4Da
1730
N III
0.76 f. 0.07
0.86 * 0.18
3d 4F-4f 4G
1750
N III
0.91 + 0.1
0.97 * 0.38; 1.04c
2065
2p2 ‘P-2p3
4D
1.5 1.5
2D
1.5
4f 2G
1.5
N III
0.82 f
0.82’; 2.6a
2p 3d ‘F”-2p
923
NIV
0.62 f 0.07
0.65b; 0.7 f 0.05d
2s 2p 3PO--2pZ 3P
1.5
955
NIV
0.33 + 0.04
0.34b
2s 2p 1Po-2p2 ‘S
1.5; 1.9
1036
NIV
0.14 + 0.08
0.35b
2s 3d ‘D-2s
1718
NIV
4.45 f 0.5
4.73c
2s 2p ‘PO-2p2 ‘D
1238
N V.
3.2 + 0.3
3.03b; 3.3a; 3.17c
1242
NV
3.3 f 0.3
3.03b; 3.3a; 3.17c
765
0.15
N III b
0.46 + 0.04
0.53”
N IVb
0.46 ~tr0.04
0.53b
4f 3Fo
1s 22s zs-2s2 2p 2PO 2s2 2p 2P0-2s 2p 2s 2s2 ‘S-2s 2p IPO
1.5; 1.9 1.5 1.5; 1.9 1.5; 0.6 1.5; 0.6
a From ref. 3. b From ref. 4. c From ref. 5. d From ref. 2. e From ref. 10.
3.2. Lifetimes. The intensity decay of the lines has been recorded by moving the foil continuously along the ion beam (at constant ion current). With our experimental arrangement, a small section of the beam (about 2 mm), is viewed by the spectrometer. This is particularly interesting for the very short lifetime
108
P. D. DUMONT
measurements. Twenty-four decay times were measured with mass-28 beams. The resulting lifetimes are given in table I. The ion-velocity calculations took into account the energy losses in the foil estimated from ref. 8. An example of a decay curve is shown in fig. 3 plotted on a semilogarithmic scale. Most of the decay curves showed effects of cascades and were decomposed into two (or sometimes three) exponential functions by a graphical method. The experimental background was always subtracted before the decomposition was done. 4. Conclusion. We have developed an apparatus to study atomic spectra with the beam-foil technique. As a fist step we studied nitrogen spectra between 650 and 2000 A. Twenty-four lifetimes were measured and these values are in agreement with other experimental results, except for N II 766 A and 901 A, N III 772 A and N IV 1036 A. In the future, we hope to improve the precision of our results by developing a pulse-counting technique for the intensity measurements.
REFERENCES 1) Beam-Foil Spectroscopy, S.Bashkin, ed., Gordon and Breach (New York, 1968). 2) Heroux, L., Phys. Rev. 153 (1967) 156. 3) Berry, H. G., Bickel, W. S., Bashkin, S., Desesquelles, J. and Schectman, R.M., J. Opt. SOC. Amer. 61 (1971) 947. 4) Buchet, J.P. and Poulizac, M.C., Proceedings of the 2nd European Conference on BeamFoil Spectroscopy and Connected Topics (1971). 5) Desesquelles, J., These, Faculte des Sciences, Lyon 1970. 6) Kay, L., Proc. Phys. Sot. 85 (1965) 163. 7) Berry, H.G., Martinson, I. and Bromander, J., Phys. Letters 31A (1970) 521. 8) Northcliffe, L.C., Annu. Rev. nucl. Sci. 13 (1963) 89. 9) Allison, R., Burns, J. and Tuzzoiino, A.J., J. Opt. Sot. Amer. 54 (1964) 747. 10) Lawrence, G.M. and Savage, D.B., Phys. Rev. 141 (1966) 67.
BEAM-FOIL
LIFETIME
MEASUREMENTS
IN THE VACUUM ULTRAVIOLET
OF NITROGEN
SPECTRAL
RANGE
P.D. DUMONT Universite’ de Litige, Institut de Physique Nucliaire, Sart-Tilman - 4000 LiPge 1, Belgique
Received 21 February 1972
Synopsis Nitrogen spectra between 650 and 2OOOA have been observed for particle energies between 0.3 and 2 MeV. Lifetimes of 24 excited levels are given.
1. Introduction. The foil excitation technique allows us to obtain lifetimes of excited states of atomic elements1*2). This paper describes beam-foil measurements of nitrogen-multiplet lifetimes in the wavelength range 650 to 2000 A ; in this wavelength region only a few experiments have been carried out previously2*3,4). The beam-foil experiments were performed at beam energies between 300 keV and 2 MeV. 2. Experimental procedure. The experimental arrangement is shown in figs. la and lb. The 2 MeV Van de Graatf accelerator of the “Institut de Physique Nucltaire - Universite de Liege” is equipped with a conventional radio-frequency ion source. After traversing the magnet, the analysed ion beam iv passed through two collimator holes (8 mm and 9 mm in diameter), and enters the “excitation chamber” which contains the carbon foil*. An externally controlled mechanical system allows the foil to be moved along the ion beam. The light emitted perpendicular to the beam is measured with a Seya-Namioka vacuum ultraviolet spectrometer designed at the “Institut d’Astrophysique - Liege”. The spectrometer entrance slit is located in the excitation chamber, 2 cm from the ion-beam axis. Slit widths between 0.1 and 0.3 mm were generally used. The spectrometer is equipped with a 1200 l/mm concave grating which has focal length of 0.5 m and is blazed at 1700 A in the first order. The photocathode of the photomultiplier (P.M.) tube (ASCOP E.M.R. 541-N-05M-14) is cov* It in fact contains 11 carbon foils, 10 mm in diameter, 10 yg/cm2 thickness, mounted on a rotating disc for the purpose of quick interchange. 104
BEAM-FOIL
LIFETIME
MEASUREMENTS
OF NITROGEN
105
ered with sodium salicylate9). The P.M. tube is connected to a d.c. amplifier, followed by a strip-chart recorder. The residual pressure in the excitation chamber is kept lower than 10e6 torr’ by an oil diffusion pump trapped with liquid nitrogen. The excitation chamber, electrically isolated, is in fact a big Faraday cup, and hence we are able to obtain the ion-beam current (relative values). This current (appr. 5 PA) is continuously recorded for the monitoring purpose.
ion beam
@rJ source
il
Van de Graaff accelerator
Fig. 1 a
.
SOP - Namiika sveclromolor
Beam foil spectroscopy_ -,L&z;rk
Fig. 1 b
‘:7,3. Experimental results. 3.1. Wavelengths. Spectra of the light emitted perpendicular to the beam were recorded at constant ion current, with a mass-28 beam. Wavelength calibration was obtained by observation of lines of known * 8 x lo-’ torr, measured with Leybold’s “Ionisations-Vakuummeter
1M;VI”.
106
P. D. DUMONT
oa
I
z 0
Fig. 2
Fig. 3a
Fig. 3b
BEAM-FOIL
LIFETIME
MEASUREMENTS
OF NITROGEN
107
origin (e.g., Ly LY,C I 1561 A, . . .). Line identifications were obtained with the aid of nearby lines whose wavelengths were listed in the literature4*5), and with the intensity variations of lines with energy6). The spectra showed many known nitrogen lines (N I-N V) as well as many unreported lines; some of these we identified as C I and C II lines, perhaps from carbon atoms ejected from the foil’). Fig. 2 is an example of such a spectrum. TABLE I 2
Ion
(A)
Energy
T
z
(ns) our results
W Other results
Transition
observed (MW
1134
NI
7.6 + 0.7
7.2 + 0.7”; -7.1b
2ps%J-2p44P
1200
NI
2.35 + 0.2
2.35c; 2.4O; 2.5=
2ps w-2p2
1242
NI
2.27 + 0.2
2.6a; 2.2=; 2.65”
2p3 ‘Do-3s
776
N II
0.33 + 0.05
0.6b
836
N II
0.51 f 0.07
0.57b
2p3 3Do-2p2 3s 3P
0.5
901
N II
0.24 + 0.035
0.65b
2p3 3D0-2p 4p 3P
0.5
916
NII
0.84 f 0.08
0.77b
2p2 JP-2ps 3PO
1345
N II
1.46 + 0.2
1.7c; 2.1s
686
N III
0.15 * 0.03
0.17 * 0.03d; 0.22b
772
N III
0.25 f 0.05
0.51b 0.80b
2p2 ‘D-2p3
2p3 jD”-3p
0.5 3s 4P
2D lD”
3D
2s2 2p ZPO-2s 2p2 *P
0.5 0.5 0.5; 1.5
0.5; 1.5 1.5 1.5
\ 2s 2p2 4P-2pJ %O \ 2s 2p2 ‘D-2p3 2Po
1.5
2s 2p2 2D-2p3 *Do
1.5 1.5
980
N III
0.79 + 0.08
991
N III
2.6 f 0.3
2.2b; 2.4 + 0.24d
2s’ 2p ‘PO-2s 2p2 *D
1184
N III
0.44 + 0.05
0.4b; 0.45 + 0.1a;
2s 2p* ZP-2p3 2PO
1.5; 0.5
0.4Sc 1324
N III
1.32 + 0.26
1.6 + 0.1”
3p 4Da
1730
N III
0.76 f. 0.07
0.86 * 0.18
3d 4F-4f 4G
1750
N III
0.91 + 0.1
0.97 * 0.38; 1.04c
2065
2p2 ‘P-2p3
4D
1.5 1.5
2D
1.5
4f 2G
1.5
N III
0.82 f
0.82’; 2.6a
2p 3d ‘F”-2p
923
NIV
0.62 f 0.07
0.65b; 0.7 f 0.05d
2s 2p 3PO--2pZ 3P
1.5
955
NIV
0.33 + 0.04
0.34b
2s 2p 1Po-2p2 ‘S
1.5; 1.9
1036
NIV
0.14 + 0.08
0.35b
2s 3d ‘D-2s
1718
NIV
4.45 f 0.5
4.73c
2s 2p ‘PO-2p2 ‘D
1238
N V.
3.2 + 0.3
3.03b; 3.3a; 3.17c
1242
NV
3.3 f 0.3
3.03b; 3.3a; 3.17c
765
0.15
N III b
0.46 + 0.04
0.53”
N IVb
0.46 ~tr0.04
0.53b
4f 3Fo
1s 22s zs-2s2 2p 2PO 2s2 2p 2P0-2s 2p 2s 2s2 ‘S-2s 2p IPO
1.5; 1.9 1.5 1.5; 1.9 1.5; 0.6 1.5; 0.6
a From ref. 3. b From ref. 4. c From ref. 5. d From ref. 2. e From ref. 10.
3.2. Lifetimes. The intensity decay of the lines has been recorded by moving the foil continuously along the ion beam (at constant ion current). With our experimental arrangement, a small section of the beam (about 2 mm), is viewed by the spectrometer. This is particularly interesting for the very short lifetime
108
P. D. DUMONT
measurements. Twenty-four decay times were measured with mass-28 beams. The resulting lifetimes are given in table I. The ion-velocity calculations took into account the energy losses in the foil estimated from ref. 8. An example of a decay curve is shown in fig. 3 plotted on a semilogarithmic scale. Most of the decay curves showed effects of cascades and were decomposed into two (or sometimes three) exponential functions by a graphical method. The experimental background was always subtracted before the decomposition was done. 4. Conclusion. We have developed an apparatus to study atomic spectra with the beam-foil technique. As a fist step we studied nitrogen spectra between 650 and 2000 A. Twenty-four lifetimes were measured and these values are in agreement with other experimental results, except for N II 766 A and 901 A, N III 772 A and N IV 1036 A. In the future, we hope to improve the precision of our results by developing a pulse-counting technique for the intensity measurements.
REFERENCES 1) Beam-Foil Spectroscopy, S.Bashkin, ed., Gordon and Breach (New York, 1968). 2) Heroux, L., Phys. Rev. 153 (1967) 156. 3) Berry, H. G., Bickel, W. S., Bashkin, S., Desesquelles, J. and Schectman, R.M., J. Opt. SOC. Amer. 61 (1971) 947. 4) Buchet, J.P. and Poulizac, M.C., Proceedings of the 2nd European Conference on BeamFoil Spectroscopy and Connected Topics (1971). 5) Desesquelles, J., These, Faculte des Sciences, Lyon 1970. 6) Kay, L., Proc. Phys. Sot. 85 (1965) 163. 7) Berry, H.G., Martinson, I. and Bromander, J., Phys. Letters 31A (1970) 521. 8) Northcliffe, L.C., Annu. Rev. nucl. Sci. 13 (1963) 89. 9) Allison, R., Burns, J. and Tuzzoiino, A.J., J. Opt. Sot. Amer. 54 (1964) 747. 10) Lawrence, G.M. and Savage, D.B., Phys. Rev. 141 (1966) 67.